No. 65: Sep-Oct 1989
The presence of high iridium concentrations at the Cretaceous-Tertiary (K-T) boundary, some 65 million years ago, has led to the widely accepted notion that an extraterrestrial projectile slammed into the earth at that time, wreaking geological and biological havoc. But the K-T boundary is anything but simple chemically and paleontologically. To illustrate, J.L. Bada and M. Zhao have found unusual amino acids in sediments laid down before and after this geological time marker.
"They find that Danish sediments spanning the narrow boundary layer contain two amino acids, alpha-aminoisobutyric acid and isovaline, that are relatively uncommon in biological materials but abundant in the organicrich meteorites. They suggest that the body which collided with Earth 65 million years ago and left the telltale iridium residue may have been organic-rich, perhaps like a C-type asteroid or a comet. Such a possibility has interesting implications for the extinction and related atmospheric effects, and supports the idea that impact events could have supplied the Earth during a much earlier period with the raw materials for organic chemical evolution."
Actually, the above quotation is pretty much in line with present mainstream thinking. Perhaps so, but Bada and Zhao identified two troubling anomalies. First, the amounts of amino acids found were surprisingly high. How could these complex molecules survive the searing temperatures engendered by high-velocity impact? Second, the amino acids may be abundant tens of centimeters above and below the K-T boundary clay containing the iridium, but they are virtually absent in the clay itself!
(Cronin, John R.; "Amino Acids and Bolide Impacts," Nature, 339:423, 1989, Also: Monastersky, R.; "Rare Amino Acids Support Impact Theory," Science News, 135:356, 1989.)
Reference. Chemical anomalies at stratigraphic boundaries are cataloged in ESC1 in Anomalies in Geology. To order this volume, visit: here.